The present invention relates generally to 5-wire touch screens, and more particularly to systems and methods for accurately determining touch pressure/force applied on 5-wire touch screens.
FIG. 1 shows an exploded isometric diagram of a conventional 5-wire resistive touch screen 10 including a transparent bottom layer 14, coated with resistive film 16 and four conductive corner pads 15-1, 15-2, 15-3 and 15-4 that can be connected to an outside contact terminal, and a top layer 12. (The layers need to be transparent to allow display or LCD (liquid crystal display) backlighting to pass through.) FIG. 2 shows a section view of an implementation of the assembled version of the exploded view of touch screen 10 in FIG. 1, wherein top layer 12 typically is formed of polyester or polyethylene terephthalate (PET) and is coated underneath with highly conductive (e.g., metal) transparent material to form wiper layer 11 (also referred to simply as “wiper 11”).
Transparent bottom layer 14 also is formed of PET, coated with transparent resistive film 16, which usually is ITO (indium tin oxide).
Elastic and insulative spacers 22 separate top layer 12 from bottom layer 14 so as to maintain a thin air gap 23 between them. Spacers 22 are typically very thin, and are used to avoid a large difference in the touch point contact resistance, which is dependent on where the touch is located relative to the locations of the spacers, and also to avoid substantial variation in the “feel” for various locations of the touch point relative to the spacers.
Applying a touch pressure to the outer surface of top layer 12 pushes a small touch contact area of wiper 11 against resistive ITO layer 16. When no touch pressure is present on top layer 12, it is separated from the bottom resistive layer 14 by spacers 22 and air gap 23.
The pressure of a touch on the upper surface of touch screen 10 typically is detected by a conventional 5-wire touch screen controller that controls various drive signals applied to the passive resistance of resistive layer 16 so as to facilitate measurement of various voltages resulting from touching various locations on the top surface of touch screen 10.
FIG. 3 shows an equivalent circuit of the idealized 5-wire resistive touch screen 10 depicted in FIGS. 1 and 2. Transparent resistive layer 16 of FIG. 2 is represented in FIG. 3 as a rectangular grid of equivalent resistors having conductive terminals UL, UR, LL, and LR on its upper left, upper right, lower left, and lower right corners corresponding to conductive pads 15-1, 15-2, 15-4, and 15-3, respectively, in FIG. 1. Wiper layer 11 thus is directly over resistive layer 16 and is connected to wiper contact terminal 35. Conductors or corner terminals 15-1, 15-2, 15-4, 15-3, and wiper contact terminal 35 are the 5 accessible conductors or “wires” of 5-wire touch screen 10. If corner terminals UL and LL are connected by a conductor 27 and terminals UR and LR are connected by a conductor 29 as indicated in the lower portion of FIG. 3, then resistive layer 16 appears as a resistor connected between conductors 27 and 29, as shown in the simplified equivalent circuit representation in the lower portion of FIG. 3. Similarly, if terminals UL and UR are connected together by conductor 26 and terminals LL and LR are connected together by conductor 28, resistive layer 16 appears as a resistor connected between conductors 26 and 28. A touch point area 31 on top conductive wiper 11 conducts touch pressure to a point or area 30 on the resistive grid when a touch is applied on touch screen 10.
FIG. 4 shows an equivalent circuit similar to the equivalent circuit shown in FIG. 3 but further including the “touch resistance” 33 having a value RZ of a touch between contact area 31 on wiper 11 and contact area 30 on ITO resistive layer 16. Touch contact areas 30 and 31 are small contact areas that occur as a result of touch pressure applied on top layer 12 that presses small area 31 of wiper layer 11 against small area 30 of resistive layer 16. Note that wiper layer 11 is assumed to be of zero resistance in the equivalent circuit of FIG. 4.
Unfortunately, it is not presently practical to provide a highly conductive (e.g., metal) contact wiper layer 11 that is sufficiently transparent for the LCD backlighting applications in which touch screens often are utilized. The wiper layer coat 11 on the lower surface of top layer 12 is presently composed of nearly-transparent ITO resistive material, the same as resistive layer 16 on the upper surface of bottom layer 14. As the result, the equivalent circuit of a practical 5-wire touch screen 10 may be as shown in FIG. 5, where resistance 34 having a value Rwiper represents the resistance of ITO resistive wiper layer 11 between the touch area 31 and wiper contact terminal 35.
Typically, each of the two ITO resistive layers 11 and 16 is approximately 90% transparent. Therefore, the top and bottom layers 12 and 14 together are 90%×90%=81% transparent, theoretically. This is very important, because lower transparency of the touch screen causes more power to be dissipated in the LCD backlighting circuitry in order to provide sufficient light intensity.
FIG. 6A is an equivalent circuit that is useful in explaining the process of determining the y-coordinate of a touch on a conventional 5-wire resistive touch screen. Measurement of the y-coordinate includes applying a voltage VDD of voltage source 38 between conductor 26, which is connected to terminals UL (15-1) and UR (15-2), and conductor 28, which is connected to terminals LL (15-4) and LR (15-3). Sensing the y-coordinate location of the electrical contact at the touch point (not shown) is accomplished through conductive terminal 35 of wiper 11. Similarly, FIG. 6B is an equivalent circuit useful in explaining the process of determining the x-coordinate of a touch on the touch screen. Measurement of the x-coordinate includes applying a voltage VDD between conductor 29, which is connected to terminals LR and UR, and conductor 27, which is connected to terminals UL and LL. Sensing the location of the electrical contact at the touch point is accomplished through conductive point 35 of wiper 11.
More specifically, the above-mentioned touch screen controller to which touch screen 10 is coupled first applies the screen driving voltage VDD of voltage source 38 between conductors 26 and 28, causing current to flow uniformly across the screen from top to bottom in FIG. 6A. The y-coordinate voltage VY is read from contact terminal 35 of wiper 11, and is given by the expression
                                          V            Y                    =                                                    V                DD                                            R                Y                                      ×                          R                              Y                ⁢                                                                  ⁢                2                                                    ,                            Eq        .                                  ⁢        1            where the y-direction resistance RY between conductors 26 and 28 is a known value that can be easily measured. RY2 is the resistance between the touch point 30 and the negative (−) terminal of voltage source 38. (RY and RY2 are illustrated in FIG. 7A.)
Similarly, the touch screen controller applies the screen driving voltage VDD of voltage source 38 between conductors 29 and 27 in FIG. 6B, causing current to flow uniformly across the screen from right to left. The x-coordinate voltage VX is read from contact terminal 35 of wiper 11, and is given by the expression
                                          V            X                    =                                                    V                DD                                            R                X                                      ×                          R                              X                ⁢                                                                  ⁢                2                                                    ,                            Eq        .                                  ⁢        2            where the x-direction resistance RX between conductors 27 and 29 is a known value that can be easily measured. RX2 is the resistance between the touch point 30 and the negative (−) terminal of voltage source 38. (RX and RY2 are illustrated in FIG. 7B.)
In addition to the foregoing touch screens, the closest prior art is believed to also include U.S. Pat. Nos. 6,246,394 and 7,215,330. U.S. Pat. No. 6,246,394 “Touch screen Measurement Circuit and Method”, issued Jun. 12, 2001 to Kalthoff et al., discloses a 4-wire touch screen digitizing system, and presents a method that measures the x and y coordinates of a touch location. U.S. Pat. No. 7,215,330 “Touch-Sensitive Surface Which Is Also Sensitive to Pressure Levels”, issued May 8, 2007 to Rantet, discloses a 4-wire touch screen that includes orthogonal conductive tracks 6 and 8 connected to resistive strips along edges of the two screens that make up the touch-sensitive screen, such that the x and y coordinates and the applied pressure can be measured. The method measures the pressure or third or “z” coordinate of a touch point on a 4-wire resistive touch screen.
Touch screen users may occasionally bump a nearby article that imparts mechanical vibration to a touch screen that can cause the associated touch screen system to erroneously interpret touch location or erroneously interpret the vibration as an intentional touch. Also, users may inadvertently touch the screen surface. If the touch screen and associated controller have the capability of measuring the touch resistance between the wiper layer and the resistive layer of the touch screen, then a “sensitivity” threshold value can be established which prevents erroneous touch interpretation due to mechanical vibration or light extraneous touches on the touch screen surface. In some applications, for example, interpreting Chinese characters being written on a touch screen or drawing of graphical features, varying amounts of force/pressure applied to the touch screen surface by a stylus can be interpreted as representing lines of varying width or darkness. Also, there are applications in which the above-mentioned sensitivity threshold value can be utilized to prevent electrical noise, such as EMI (electro-magnetic interference), from causing touch interpretation errors.
The prior 5-wire touch screen systems can only measure x- and y-coordinates, lack any method for obtaining third-coordinate or pressure data, and have limited capability for performing certain functions, such as signature verification, in which the pressure applied to provide a valid signature can be very significant. Without the pressure measurement of the present invention for a 5-wire touch screen system, the 5-wire touch screen system can generate only 2-dimensional coordinates, and therefore supports only 2-dimensional applications on the touch screen surface.
Thus, there is an unmet need for a system that measures 3 touch point coordinate voltages developed in a touch screen panel to represent x coordinates, y coordinates, and a touch point contact resistance coordinate, respectively, between a wiper layer and a resistive layer of a 5-wire touch screen.
There also is an unmet need for a system that measures 3 touch point coordinate voltages developed in a touch screen panel to represent x coordinates, y coordinates, and a touch point contact resistance coordinate between a wiper layer and a resistive layer of a 5-wire touch screen, wherein the touch point contact resistance is utilized to determine a touch point contact pressure or force.
There also is an unmet need for a touch screen system capable of providing improved signature verification by utilizing the touch pressure contact resistance in a 5-wire touch screen.
There also is an unmet need for a touch screen system capable of providing touch intensity measurements by utilizing the touch point contact resistance on a 5-wire touch screen.
There also is an unmet need for a touch screen system capable of providing touch sensitivity measurements by utilizing the touch point contact resistance on a 5-wire touch screen, wherein EMI (electro-magnetic interference) from the touch screen can be distinguished from real touches or pressures.
There also is an unmet need for a touch screen system capable of providing touch sensitivity measurements by utilizing the touch point contact resistance in a 5-wire touch screen. wherein touch point size information can be determined.